The unprecedented scale of national infrastructure development has brought forward higher requirements for high performance concrete, and also posed a great challenge. The complex for structure of constructions and automation of construction methods require that the fluidity of high performance concrete to hold longer, and the high performance concrete itself, using a lot of mineral admixtures and additives with a variety of functions also makes it harder to maintain the fluidity. Moreover, due to the vast territory and significant seasonal variation of China, the quality and properties of concrete raw materials vary greatly, the quick loss of fluidity for fresh concrete during mixing and transporting often encounters. The insufficient capability for slump retention of concrete has brought a lot of problems for project construction and quality of concrete, especially in the summer, under high temperature, when the slump of concrete is relatively low or there is the demand for super long time slump, this phenomenon becomes even more pronounced. The reasons are as follows: the quick loss of slump was caused by the acceleration of cement hydration and water evaporation due to high temperature in summer; concrete with relatively low slump may encounter fluidity loss in short time due to its low content of free water in the system; the prolonged time for concrete construction has led to the increase of cement hydration products, and the fluidity loss may gradually occur if the generated hydration products are not going to be dispersed with extra dispersant in the concrete system. Therefore, the natures of fluidity loss caused by a variety of factors are not the same, and the time requirements for supplement of dispersant are not the same either.
With the advantages of low dosage, high water reduction, better slump retention than naphthalene superplasticizer, reducing concrete shrinkage, no pollution during preparation and so on, the polycarboxylate superplasticizer has become an indispensable key ingredient in preparing high performance concrete and also become a hot research topic. After the rapid development in recent years, the polycarboxylate superplasticizer has been greatly improved in water reducing capability, which indirectly led to the gradual decrease of the dosage for polycarboxylate superplasticizer in concrete system, and also gradually exposed the insufficient slump retention capability of the concrete mixed with polycarboxylate superplasticizer.
In order to solve the issues quick fluidity loss of concrete mixed with polycarboxylate superplasticizer, researchers in related fields at home and abroad have done a lot of research. In addition to traditional methods of retarding, currently the most effective method to ease the concrete fluidity loss is to add slump-retaining component, and many patents have published the preparation methods of the associated polycarboxylate polymers with the function of slump retention.
Patent CN101786824A discloses the preparation method of a sustained-release polycarboxylate water-reducing agent, this method is made through the radical aqueous copolymerization of polyoxyethylene ether macromonomer containing unsaturated double bond, (methyl) acrylic acid, unsaturated sulfonic acid or its monomer, monoolefins acid derivatives and diene carboxylic acid derivatives, and then neutralized with alkaline solution. The polycarboxylate water-reducing agent prepared through this method has the effect of preventing the fluidity loss due to long time transportation, however, the no dispersed effect at initial stage may cause the issue of high dosage when using as complex together with general polycarboxylate water-reducing agent, and the polymer in this structure needs longer time before it can have the effect of slump retention, therefore, it cannot solve the problem of quick fluidity loss due to the acceleration of cement hydration under high temperature in summer, which has significant limitation.
Patent CN101357833A discloses a slump-retaining type polycarboxylate superplasticizer; this method adopts part of the unsaturated polyether macromonomer to replace the unsaturated macromonomer to improve the stability of the molecular structure of polymer under the strong alkaline environment of cement, so as to improve the capability of slump retention. Meanwhile, itaconic acid was introduced into polymer molecular as adsorption group to improve its capability of slump retention. The polycarboxylate superplasticizer prepared through this method has stronger dispersion capacity, but due to no sustained-release groups in the molecule, it is unable to meet the needs of later age dispersion of concrete system, thus, there is still great room for improvement.
Patent CN102093521A discloses the preparation method of a polycarboxylate high slump-retaining agent; this method adopts the copolymerization of polyethylene glycol allyl polyether, maleic anhydride, maleic anhydride polyethylene glycol diester and acrylic acid, and then the neutralization by adding alkali. This method took the advantage of not requiring water carrier during the esterification of anhydride and reduced pollution; however, due to the low activity of polymerization of allyl polyether, maleic anhydride and its derivatives, there is a certain degree of difficulty for the polymerization, so that there is still great room for improvement.
In summary, we can see that, due to the limit of molecular structure of the existing technology, only certain aspect of the integrated problem that has been encountered during the use of concrete can be met, such as, most polymers with long slump-retaining time have relatively low water reduction, and cannot present the effect of slump-retaining in short time, thus cannot meet the requirement of some concretes for slump-retaining in short time, while the slump-retaining agent which can meet the short-term needs cannot last long enough. Therefore, it is very necessary to develop a fully functional slump-retaining type polycarboxylate superplasticizer of high performance, which can quickly present its effect and can remain functional, to solve the issue of the fluidity loss of modern high performance concrete.